A new polystyrene–TiO2 nanocomposite film and its photocatalytic degradation

Applied Catalysis A: General 264 (2004) 237–242

A new polystyrene–TiO2 nanocomposite film and its photocatalytic degradation Ling Zan∗ , Lihong Tian, Zhongshi Liu, Zhenghe Peng College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China Received 9 August 2003; received in revised form 23 December 2003; accepted 26 December 2003

Abstract A new kind of photodegradable polystyrene (PS)–TiO2 nanocomposite film was synthesized. The TiO2 nanoparticles were first modified by grafting polymer on its surface (G-TiO2 ), and then the PS–G-TiO2 composite films were formed through polymerization. The photocatalytic degradation of the composite films was investigated. The as-prepared films were characterized by scanning electron microscope (SEM), gel permeation chromatogram (GPC), FT-IR and UV-Vis spectroscopy, and the photoinduced weight-loss. The results show that PS–G-TiO2 nanocomposite films could be efficiently photocatalytically degraded under UV illumination in air. The weight-loss of the PS–G-TiO2 film reached 29%; the average molecular weight reduced to 1/4 of the original molecular weight under UV-light irradiation for 300 h, implying that some benzene rings had been cleaved. The photocatalytic degradation mechanism of the films is briefly discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: TiO2 ; Polystyrene; Nanocomposite film; Solid-phase photocatalytic degradation

1. Introduction As a conventional plastic material, a large amount of polystyrene (PS) and the expanded polystyrene (EPS) is used in food service and retail industry. In addition, PS foam packaging is widely used to protect electronic instruments, household appliances, auto parts, and other fragile goods from damage. Due to its inertness, polystyrene and related plastic products are non-biodegradable in natural environment. The waste PS plastics do not decompose in landfills, causing a serious environmental problem, the so-called “white pollution”. It is well known that TiO2 will produce electron-hole pairs under illumination of UV-light. TiO2 photocatalyst has been successfully used to purify water and air, to degrade the organic pollutants and to kill the bacteria [1–3]. Previous studies on the TiO2 photocatalytic degradation of polymers mainly dealt with liquid-phase reactions, such as photocatalytic degradation of poly vinyl chloride (PVC) particles in TiO2 suspension aqueous solution [4]. Recently, there are a few studies on the solid-phase photocatalytic degradation ∗ Corresponding author. Tel.: +86-27-8768-2919; fax: +49-2365-32192. E-mail address: [email protected] (L. Zan).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.12.046

of polymer–TiO2 composite. Only one reference is found [5]. They investigated the photocatalytic degradation of PVC–TiO2 composite films. Their PVC–TiO2 composite was prepared by directly embedding TiO2 into PVC. From the viewpoint of solid-phase photocatalytic degradation, a well-dispersed and uniformly mingled microstructure of polymer–TiO2 composite is highly desired. Usually, TiO2 particles will aggregate significantly in the low polar medium, if there is not enough steric hindrance [6,7]. The TiO2 nanoparticles incorporated into the polymer matrix appear in the form of huge agglomerates, whose size extends up to a few micrometers. The micrometer-sized agglomerates reduce the photo degradation efficiency significantly in two mechanisms [5]: (a) decreasing the interface area between polymer and the photocatalytic agent; and (b) inducing rapid whitening. The photoinduced whitening quickly shortens the light penetration depth into the composite film, which hinders further photodegradation. Nanoparticles also generate nanocavities, which scatter out little incoming light. If the TiO2 nanoparticles could get a good dispersion in the polymer, the efficiency of photocatalytic degradation is expected to improve to a great extent. In this study, a new kind of polystyrene–TiO2 (PS–TiO2 ) composite film has been synthesized, via nanoparticles grafting and polymerization. To our best knowledge, there is no

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report on the research of the photodegradable PS–G-TiO2 composite film. The processing of polystyrene–TiO2 composite film is as follows: nano-TiO2 powders were first modified by grafting polystyrene onto the surface; then the grafted TiO2 nanoparticles were dispersed in the monomer of PS. The PS–G-TiO2 nanocomposite finally formed through polymerization. This way might be applied in an industry process of the PS.

2. Experimental Four chemical reagents were used to prepare PS–G-TiO2 composite film. (1) TiO2 nanoparticles (98% anatase) were prepared in the authors’ laboratory [8,9]; the primary particle diameter is in a range of 70–100 nm. (2) The molecular formula of WD-70 silicone coupling agent (Wuhan University Chemical & New Material Co. Ltd.) is as follows:

(3) Styrene (Shanghai Chemical Reagent Co. Ltd.): washed with dilute aqueous solution of sodium hydroxide and water, dried over barium oxide and distilled under reduced pressure before use. (4) 2,2 -Azo-bis-iso-butyronitrile (AIBN) (Shanghai Chemical Reagent Co. Ltd.) was re-crystallized from methanol and dried in vacuum. The synthesis of PS–G-TiO2 composite film consists of two major steps. (i) The grafting of TiO2 nanoparticles: 5 ml WD-70 was dissolved in 125 ml isopropanol; then 50 g TiO2 (dried at 120 ◦ C for 2 h) was dispersed in the solution under ultrasonic irradiation for 20 min. This suspension solution was added in a Wolff bottle; stirring at 35 ◦ C for 2 h. 25 ml styrene, in which 0.2 g AIBN was dissolved, then was added into the Wolff bottle and the mixture was refluxed with stirring at 80 ◦ C for 20 h under nitrogen. After the reaction, the mixture was centrifuged and the grafted TiO2 was extracted three times with toluene and dried in vacuum at 110 ◦ C. The grafted TiO2 should be stored at room temperature in the dark. The structure and graft ratio of the grafted TiO2 had been reported elsewhere [10]. (ii) The preparation of composite films: the G-TiO2 particles were dispersed into styrene under ultrasonic irradiation for 20 min. The ratio of TiO2 to PS are 0.4 and 1.0 wt.%, respectively. Then AIBN(0.03 wt.%) is dissolved in the above suspension solutions and polymerization reaction is conducted at 105 ◦ C for 15 h. 30 g of this composite is dissolved in 100 ml chloroform. The viscous liquid is spread on a slide glass surface (10 cm × 10 cm) and dried in closed vessel for 2 days. In order to compare the photocatalytic activity, pure PS

film and PS–TiO2 (no grafting nano-TiO2 ) film were also prepared in a similar procedure. The thickness of these films was measured to be 40–50 ␮m by a micrometer. The photodegradation performance of different composite films is examined. These composite films are irradiated under a 30 W UV-lamp, the center wavelength of which is 254 nm. The typical size of a film sample was about 4 cm2 . Each film sample was located 10 cm away from the lamp. The photodegradation is carried out under ambient air at room temperature. The samples are weighted every 24 h during 300 h of irradiation. UV-Vis spectrophotometer (Hitachi UV-3400) and FT-IR spectrophotometer (JA Transform Nicolet FT-170SX) are used to characterize the spectral transmittance of the film before and after irradiation. The surface morphologies of the composite film are observed by scanning electron microscope (SEM, Hitachi X-650.) The average molecular weight (Mw ) of the PS films is measured by gel permeation chromatograms (GPC, Waters 2690D Separation Module, Waters 2410 Refractive Index Detector).

3. Results and discussion Fig. 1 shows the UV-Vis transmittance spectra of pure PS film and of PS–G-TiO2 composite film. The transmittance patterns of the two kinds of film are obviously different. In the UV region, the transmittance of PS–G-TiO2 films is much lower than that of pure PS films due to the UV absorption of TiO2 nanoparticles. In the visible light region, the transmittance of pure PS films was about 90% at 700 nm. The transmittance does not decrease with increasing UV-light illumination time. The transmittance of PS–G-TiO2 (1 wt.%) composite films is about 80% at 700 nm; this decreases regularly and significantly with increasing the UV-light illumination time (Fig.1(b)). The difference is due to the fact that TiO2 nanoparticles dispersed in the films produce many photogenerated electrons and holes under UV-light illumination. These electrons and holes will degrade PS to form some cavities in the films, which cause the scattering of light. So, a low transmittance was observed in this film with increases in the UV-light irradiation time. Fig. 2 shows the FT-IR spectra of the pure PS films and PS–G-TiO2 composite films. The bands in the region 1700–1710 cm−1 are the characteristic absorptions of carbonyl group ν (C=O). It can be seen that the intensities of carbonyl peak increase continually with increasing the illumination time in PS–G-TiO2 composite films (Fig. 2(b)), and the peak intensity does not change any more when the irradiation time reached 30 h in pure PS film (Fig. 2(a)). This phenomenon clearly proves that the presence of TiO2 nanoparticles in polymer films promotes the photocatalytic oxidation of polystyrene films. The formation of the carbonyl groups in the photooxidized pure PS has been well

100

100

80

80 T ran sm it t a nc e ( % )

Trans mi tt an ce (%)

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60

0h 15 h 30 h 45 h

40

(a)

500

600

700

800

(a)

1600 −1

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80

80 Tran sm itt a nc e (% )

70 Tr ans mi tt anc e (%)

1800 Wave number / cm

W a ve length / nm

90

60 50 40 30

0h 15 h 30 h 45 h

20 10 300

400

500

600

700

60

40

0h 15 h 30 h 45 h

20

0 2000

0

(b)

0h 15 h 30 h 45 h

40

0 2000

0 400

60

20

20

300

239

800

Wa vel engt h / n m

Fig. 1. UV-Vis transmittance spectra of (a) pure PS films and (b) PS–G-TiO2 (1.0 wt.%) films.

documented elsewhere [11]. In the composite film, the surface water on TiO2 react with valence band (VB) holes to generate hydroxyl radicals or O2 molecules react with conduction band (CB) electrons to generate reactive oxygen species; these radicals and species are responsible for most of the oxidizing power of TiO2 photocatalysts. These hydroxyl radicals and active oxygen species oxide the C–H bond in polymer chain to form the carbonyl group. The detailed mechanism is being investigated. Fig. 3 shows the surface morphologies of polystyrene film, PS–TiO2 and PS–G-TiO2 composite film before and after irradiation. It can be observed from Fig. 3(b) and (c) that the G-TiO2 nanoparticles were well dispersed in the composite film. Compared to the unmodified TiO2 particles, the size of G-TiO2 particles in the film are about 100–300 nm, but the non-grafted TiO2 particles are in the range of 500–600 nm in the same concentration range of TiO2 . The finer particle results in a larger interface area between polymer and catalyst and a higher photocatalytic activity [5]. Fig. 3(d) and (e) shows the films image under UV illumination after 300 h. In PS–G-TiO2 composite film, big cavities were

(b)

1800 Wavenumber / cm

1600 −1

Fig. 2. FT-IR spectra of (a) pure PS films and (b) PS–G-TiO2 (1 wt.%) films.

formed not only on the surface, but also inside the films. The film structure was destroyed, and the chalking phenomenon took place. On the other hand, only small cavities and cracks were observed on the surface of the pure polystyrene film. This indicates that TiO2 nanoparticles will greatly enhance the photocatalytic degradation of the polystyrene material. Fig. 4 shows the photodegradation weight loss curves of the films. Obviously, the weight loss rates of the TiO2 incorporated films were much higher than that for the simple polystyrene film. The weight losses of PS–G-TiO2 and PS–TiO2 composite films reach 29 and 13%, respectively, while that of pure PS films reached only 7% under UV-light illumination after 300 h. Because the G-TiO2 nanoparticles were more uniformly dispersed in polymer films, the weight loss of PS–G-TiO2 films was much higher than that of PS–TiO2 films, indicating the PS–G-TiO2 films have a higher photocatalytic activity. These results agree well with SEM observations. It can also be seen from Fig. 4 that the weight loss rates of PS–G-TiO2 films increased with increasing TiO2 concentration. It is interesting to note that the weight percentage of benzene ring in the polystyrene molecule is about 74%, and the

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Fig. 3. SEM images of composite films: (a) pure PS film before irradiation, (b) PS–G-TiO2 (1.0 wt.%) film before irradiation, (c) PS–TiO2 (1.0 wt.%) composite film before irradiation, (d) pure PS film illuminated for 300 h, and (e) PS–G-TiO2 (1.0 wt.%) composite film illuminated for 300 h.

weight loss of PS–G-TiO2 composite film reaches 29% under UV-light illumination after 300 h. These data may imply that some benzene rings were cleaved. Under UV-light irradiation, the photolysis of the pure polystyrene film occurs in air due to benzene ring in

polystyrene molecule being stimulated by absorbing UV light with wavelength less than 280 nm and forming the macromolecule radical. These radicals react further with O2 , leading to the chain cleavage. The efficiency of this photolysis process, however, is very low. The detailed

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30

Weight loss (%)

Table 1 The variation of molecular weight of polymer films with irradiation time by GPC

pure PS film PS-TiO2(1wt%) film PS-G-TiO2(0.4wt%) film PS-G-TiO2(1wt%) film

25

241

20 15 10

Sample numbera

Mn b /103 g mol−1 )

Mw /103 g mol−1 )

Polydispersity

1(PS-G-TiO2 )

208

377

1.81

2(PS-G-TiO2 ) 3(pure PS film) 4(pure PS film)

45 102 78

109 293 227

2.41 2.87 2.91

a

5

b

No. 1: before illumination; no. 2, no. 4: after 300 h illumination. Mn : number average molecular weight.

0 0

50

100

150

200

250

300

Illumination time / h Fig. 4. Weight loss curve of pure PS, PS–TiO2 and PS–G-TiO2 composite films under UV illumination in air.

degradation, the biological degradation is possible to be applied in this case to degrade furthermore the polystyrene composite film [16,17].

mechanistic discussion on the direct photolysis of PS can be found in the literature [11]. The initiation in the photocatalytic degradation of PS–TiO2 composite film can be quite different. According to the literature [2,12], TiO2 molecules were stimulated by absorbing UV light whose wavelength is lower than 390 nm, to generate various active oxygen species as follows:

4. Conclusions

The photocatalytic degradation of the PS matrix in the solid-phase is initiated by active oxygen species such as O2 •− , HOO• , HO• forming on the surface of TiO2 , like many TiO2 liquid and gaseous phase photocatalytic reactions [13–15]. These active oxygen species attack neighboring polymer chains to abstract a hydrogen atom and form the carbon-centered radicals such as –CH2 • CC6 H5 –; their successive reactions with O2 •− , HOO• and HO• produce hydroxyl derivatives and carbonyl intermediates with the result that the chains are cleaved. Finally, carbon dioxide is evolved [4]. The photocatalytic degradation of the PS films is accompanied by the reducing of PS molecular weight, which is measured by GPC (as shown in Table 1). The Mw of PS–G-TiO2 films decreases from 377 × 103 to 108 × 103 g mol−1 after 300 h of illumination, which means that they have been decreased to almost 1/4 of the original molecular weight. A similar analysis for the pure PS films shows that the Mw decrease to about 3/4 of the original molecular weight. This may imply that the bond scission in the backbone of the polymer in PS–G-TiO2 composite films is caused by both the direct photolytic and the TiO2 photocatalytic reactions. With this initial photocatalytic

(ii) The organic modification by grafting polymer on the surface of TiO2 particles has been proved to be an effective process to obtain good dispersion of TiO2 nanoparticles in polymer films. (iii) After 300 h of UV-light illumination, the weight-loss of the composite film reached 29%; and the average molecular weight (Mw ) decreased to 1/4 of the original Mw . The PS–G-TiO2 composite is a hopeful new environment-friendly polymer material.

(i) The presence of TiO2 nanoparticles in polystyrene films greatly promotes the photocatalysis degradation of the composite. Furthermore, if the particles are well dispersed in the polymer matrix, the efficiency of the photodegradation can be significantly increased.

Acknowledgements This works was financed by Key Research Project (99-554-03-01) of the State Ministry of Education, China.

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